It is known that a polymer having a terminal alkenyl group crosslinks by itself or with the aid of a curing agent such as a hydrosilyl-containing compound to give a highly heat-resistant and durable cured product. As the main chain of such a polymer, there can be mentioned, among others, a polyether polymer such as polyethylene oxide, polypropylene oxide, polytetramethylene oxide, etc.; a hydrocarbon polymer such as polybutadiene, polyisoprene, polychloroprene, polyisobutylene, etc. and hydrogenation products thereof; a polyester polymer such as polyethylene terephthalate, polybutylene terephthalate, polycaprolactone, etc.; or a polysiloxane such as polydimethylsiloxane, etc., and those polymers have been used in a variety of applications depending on the characteristics of the respective skeletal structures.
Vinyl polymers have characteristics not possessed by the above-mentioned polymers, for example high weathering resistance, high heat resistance, oil resistance, transparency, etc., and those having an alkenyl side-chain have been suggested to find application as super-weather-resist-ant coating materials [e.g. Japanese Kokai Publication Hei-3-277645; Japanese Kokai Publication Hei-7-70399].
On the other hand, alkenyl-terminated vinyl polymers can hardly be synthesized and are not prominent yet on the commercial scene.
Japanese Kokai Publication Hei-1-247403 discloses a process for producing an acrylic polymer having an alkenyl group at both terminals by using an alkenyl-containing dithiocarbamate or diallyl disulfide as a chain transfer agent.
Moreover, Japanese Kokai Publication Hei-6-211922 discloses a process for producing an alkenyl-terminated acrylic polymer which comprises preparing a hydroxy-terminated acrylic polymer using an hydroxyl group-containing polysulfide or an alcoholic compound as a chain transfer agent in the first place and, then, synthesizing an alkenyl-terminated acrylic polymer by using the reactivity of the terminal hydroxyl group.
However, it is difficult to introduce an alkenyl group into the terminal position of the polymer molecule without fail by such prior-art technology. Moreover, since these processes depend on the standard radical polymerization reaction, the molecular weight distribution (the ratio of weight average molecular weight to number average molecular weight) of the product polymer is usually as broad as two or more, thus giving rise to the problem of high viscosity. High viscosity may, in turn, cause the problem that when the polymer is utilized as a sealant or an adhesive, not only difficulties in handling but also the inability to incorporate an adequate amount of a reinforcing filler is inevitable.
Furthermore, it is not easy to introduce a (meth)acryloyl group, which has radical-polymerizing activity of its own, into vinyl polymers which are produced by radical polymerization. Particularly, few (meth)acryloyl-terminated oligomers have been successfully synthesized to this day.
Meanwhile, curable elastomeric compositions have so far been used broadly as adhesives, sealants and cushioning materials. Classified by type of curing, those compositions can be roughly divided into the so-called moisture-curing compositions which are stable under tightly sealed conditions but begin to cure at room temperature by moisture when exposed to atmospheric moisture to form elastomers and those compositions which cure on heating, for example by way of hydrosilylation reaction.
In the curable composition utilizing an actinic ray including UV light and an electron beam as well as in the thermosetting curable composition, low molecular compounds having a (meth)acryloyl group are utilized on many occasions. In such cases, the malodor produced by evaporation of unreacted low-boiling components during and after curing has been a matter of serious concern. To overcome this disadvantage, (meth)acryloyl group-containing oligomers have been employed. However, mainly from synthetic considerations, the use of such oligomers is limited to the epoxy acrylate, urethane acrylate and polyester acrylate systems and, moreover, few oligomers of large molecular weight are available. As a result, they tend to yield comparatively hard products on curing, failing to give products having satisfactory rubber elasticity.
Environmental considerations have brought about changes in coating technology. The amount of volatile organic compounds (VOC) released from coatings into the atmosphere is a matter of concern. In water-based coatings, a volatile solvent is used for promoting cohesion of latex particles and assisting in film formation. This is usually carried out by preparing a polymer or copolymer dispersion having a glass transition point (Tg) not less than room temperature, plasticizing the polymer with a volatile solvent to effectively reduce its Tg and then allowing the polymer to form a film at room temperature. As, after film formation, the solvent is evaporated off, the polymer successfully applied at a temperature not more than its intrinsic Tg is left behind. In this procedure, external heating is not required for film formation. This technology works well but is losing its perennial position on account of the recent rigorous world-wide control over the VOC level in coatings.
As a means for obviating the use of a solvent, there is available a technology such that an emulsion polymer is crosslinked after removal of water. This crosslinking method leads to improvedmechanical properties of the polymer film and, under suitable conditions, to reduced requirements of the solvent for assisting in said cohesion and film formation. However, the field capable of using crosslinking a water-based coating after removal of water is limited. A compound having a radical-polymerizable group may be mixed with a photoinitiator and, then, cured by exposure to ultraviolet light. Under suitable conditions, this crosslinking occurs regardless of whether the composition is applied in hydrous state or in dry state. The resulting film finds application as a coating, a paint or a sealant.
Acrylic pressure sensitive adhesives may exhibit well-balance adhesiveness even without addition of a tackifying resin and have been produced in large quantities alongside natural rubber-type pressure sensitive adhesives. Acrylic pressure sensitive adhesives are deficient in cohesive force, in particular, because of the molecular weight and molecular weight distribution characteristics, and this disadvantage is generally overcome by crosslinking. For this crosslinking, many methods have been developed; for example the methods comprising adding various crosslinking agents such as a polyisocyanate compound, an epoxy compound, a polycarboxylic acid, a polyamine compound, a phenolic resin and a sulfur compound.
The acrylic pressure sensitive adhesive, in general, is produced by the process in which a pressure sensitive adhesive solution obtained by the solution polymerization of a vinyl monomer system predominantly composed of an acrylic monomer in an organic solvent or an emulsion obtained by the emulsion polymerization of such a monomer system in an aqueous medium is applied to a substrate by way of coating or impregnation and dried by heating.
However, the use of a pressure sensitive adhesive solution has been found to be disadvantageous in that not only a considerable energy is required for drying the solution but it entails an atmospheric pollution and has the risk for the solvent catching a fire. When an emulsion is used, too, evaporation of water requires a still larger energy than needed for evaporation of a solvent and, in terms of performance, too, the compatible monomer species are limited so that this technology is lacking in the versatility necessary for meeting a large variety of needs for pressure sensitive adhesives.
As means for obviating the above disadvantages, photopolymerizable pressure sensitive adhesives have been proposed. In many photopolymerizable compositions of this type, low molecular compounds having a (meth) acryloyl group are employed. However, the odor due to evaporation of unreacted low-boiling components during and after curing is a matter of serious concern. Moreover, in order that a composition containing a monomer as a major component may be evenly coated on a substrate, the composition needs to have a certain degree of viscosity and, therefore, must be somehow thickened.
To overcome said odor problem, oligomers having (meth)acryloyl groups have been employed. However, such oligomers are limited to the epoxy acrylate, urethane acrylate, polyester acrylate and the like chiefly from synthesis reasons and, moreover, few oligomers of high molecular weight are available.
Japanese Kokai Publication Hei-2-60981 describes a method of thickening compositions which comprises adding an acryl rubber, or an epichlorohydrin rubber but since the rubber so added remains uncrosslinked in the pressure sensitive adhesive, the performance of the pressure sensitive adhesive is unavoidably sacrificed.
In view of the above state of the art, the present invention has for its object to provide a vinyl polymer containing a terminal (meth) acryloyl group in a high proportion, a curable composition comprising the polymer, an aqueous emulsion thereof and, as end products, an adhesive composition and a pressure sensitive adhesive comprising the same.